If you use Gmail you may become frustrated when you click a
mailto: link by accident and now your desktop client of Outlook or Mail starts up.
navigator.registerProtocolHandler() (which we've covered here before) you can wire up Gmail as your default mail client for all
mailto: links in Chrome and Firefox.
navigator.registerProtocolHandler("mailto", "https://mail.google.com/mail/?extsrc=mailto&url=%s", "Gmail");
If you ever need to removing this setting, you can do that at
chrome://settings/handlers in Chrome and Preferences->Applications->mailto in Firefox.